RE: Restate fixed asset value to the net book value as of the date of ownership change
I'm assuming you're talking wide-scale changes? If so, you're probably already aware but there isn't an integration available to accomplish this. You'd have to manually open the assets that had a NBV in the 'Asset Book' window & then adjust the cost basis, LTD Deprec, and/or remaining life fields manually.
You can check out the options available in the 'Mass Change' window under Fixed Assets>>Transactions but I don't believe there's anything there that will address this situation.
Outside of that, if trying to do large scale, you'll want to look @ retiring using the 'Mass Retirement' option & then reimporting as you've put...
Wish there was an easier answer but there's a lot going on behind the scenes in Fixed Assets that making importing updates to assets tough to do.